Porous metal oxides can be prepared by a number of techniques ranging from sol-gel synthesis to various templating/support methods. These porous metal oxides have shown enhanced catalytic activity, compared to bulk material, but are still limited by surface areas less than 1000 m2/g. This is even the case when using high surface area templates such as SBA-15 or MCM-41. Surface areas for the templated metal oxides can be less than 200 m2/g. The use of supports, such as carbon nanotubes, also yields surface areas less than 300 m2/g. Another issue presented by many porous metal oxides is that their pore structure collapses at elevated temperatures. For example in titania aerogels, this lack of pore stability results in order of magnitude decreases in surface area under heating. The presence of silica has been shown to provide some stabilization of pores at high temperatures in titania-silica composites. However, the surface area is still significantly decreased under heating.
Carbon nanotubes (CNTs) possess a number of intrinsic properties that have made them promising materials in the design of composite materials. CNTs can have electrical conductivities as high as 106 Sm−1, thermal conductivities as high as 3000 Wm−1K−1, elastic moduli on the order of 1 TPa, and are extremely flexible. Unfortunately, the realization of these properties in macroscopic forms such as foams and composites has been limited. Foams, though conductive, tend to be mechanically weak due to their dependence on van der Waals forces for mechanical integrity.
The treatise, Introduction to Nanotechnology, by Charles P. Poole, Jr., and Frank J. Owens. John Wiley &. Sons, 2003, states: “Nanotechnology is based on the recognition that particles less than the size of 100 nanometers (a nanometer is a billionth of a meter) impart to nanostructures built from them new properties and behavior. This happens because particles which are smaller than the characteristic lengths associated with particular phenomena often display new chemistry and physics, leading to new behavior which depends on the size. So, for example, the electronic structure, conductivity, reactivity, melting temperature, and mechanical properties have all been observed to change when particles become smaller than a critical size.”
In addition, developing novel porous carbons and carbon composites remains important for a range of current and emerging technologies such as batteries, hydrogen storage, catalysis, and adsorbents. (References 1-8) Porous carbons are promising candidates for these applications because they possess high surface areas, are chemically stable, and have high electrical conductivities. Unfortunately, carbon has some key drawbacks that limit its performance in certain cases. Carbon has a fairly low resistance to oxidation at elevated temperatures, limiting the operating temperature of carbon-supported catalysis in an oxidative environment. A common way to improve the thermal stability of a porous carbon is to cover its inner surface with a more thermally stable material (e.g. an oxide or carbide) to serve as a barrier to oxygen diffusion. (References 5,9) Typically, thermal stability is improved but surface area is dramatically reduced. The reduction in surface area occurs because the micropores, present in large quantities in most porous carbons, are blocked by the depositing species, decreasing the accessible active sites. Furthermore, if high temperature treatment (e.g. carbothermal reduction) is used to produce a carbide coating, additional surface area is lost due to sintering. Therefore, though the thermal stability may be enhanced, the surface area can be reduced to less than half that of the original porous carbon. The design of a high surface area carbon containing hierarchical porosity (micro- and macropores) could minimize the instance of micropore blockage, providing a support that could accept deposition of a thermally stable oxide
or carbide while maintaining a high surface area.